The present invention relates to objective lens assembly with high numerical apertures and a large angular field.
An objective lens assembly is used for imaging optical radiant energy on an optically sensitive medium at high recording density, or for reading information on medium in the image space. Image space refers to the short or near conjugate space of the objective lens assembly.
For practical use with monochromatic or quasi-monochromatic light of a wavelength ranging from the ultraviolet to the near infrared, high resolving power requires an objective lens assembly numerical aperture (NA) typically of 0.55 or greater.
Objective lens assemblies of high numerical aperture for recording and reading a single channel or stream of data on optically sensitive medium are well known in the art, such as can be found with writable/readable compact disks (CD) and digital versatile disks (DVD). If the image space is in air (n=1), the greatest theoretically achievable NA is 1. Single element objective lens assemblies that utilize aspheric surfaces and high index of refraction glasses can achieve numerical apertures of approximately 0.55. However, when the numerical aperture becomes larger than 0.55, optical aberration introduced by the objective lens assembly element become too great for the objective lens assembly to be usable at such numerical apertures. Because of this problem, objective lens assemblies with high numerical apertures (i.e., numerical apertures greater than 0.55) are constructed of two or more objective lens assembly elements.
Also, focus adjustment is effected in most optical recorders by the use of a driving mechanism that moves the objective lens assembly longitudinally. Due to the high NA of the objective lens assemblies used in the current art, working distance, i.e., the separation from the last objective lens assembly surface to the optically sensitive medium, is usually very small. Small working distances place limits on the maximum accommodation for objective lens assembly focus adjustment.
A large working distance is not only desirable for focus adjustment but also to avoid objective lens assembly interference with debris attached to the medium or inadvertent scratching of the medium by the objective lens assembly.
Two element high numerical aperture objective lens assemblies have been shown to produce numerical apertures of over 0.7 and approaching 0.9. Such a high numerical aperture objective lens assembly is disclosed in the article entitled xe2x80x9cA Rewritable Optical Disk System with Over 10 GB of Capacityxe2x80x9d by Kiyoshi Osato, Kenji Yamamoto, Isao Ichimura, Fumisada Macda and Yutaka Kasami. However, the correction of objective lens assembly aberration such as coma and astigmatism away from the objective lens assembly optical axis is usually insufficient to permit high density recording and reading at any position other than near the objective lens assembly optical axis at best focus. Usually only a single channel is recorded and read at the axial position where aberrations are small and field aberrations non-existent.
The application of an aspheric surface to one or more surfaces of single and two-objective lens assembly-element objective lens assemblies for optical recording type devices is also well known in the art. Specifically, U.S. Pat. Nos. 4,595,264, 4,671,623, and 6,075,656 disclose some such objective lens assemblies employing one or more aspheric surfaces. Generally, as aspheric surfaces are added to a design, the theoretical performance of the design increases. Although the use of aspheric objective lens assembly surfaces facilitate the theoretical aberration-correction properties in optical designs like the aforementioned optical objectives, in practical applications, it is desirable to limit the number of aspheric surfaces due to objective lens assembly element manufacturing sensitivities and assembly complexities. Specifically, if more than two aspheric surfaces are used in a two element design, one of the objective lens assembly elements must be a bi-aspheric, i.e., both surfaces of at least one of the objective lens assembly elements are aspheric. Fabrication of bi-aspheric objective lens assembly elements are often difficult because the two-aspheric halves must be very accurately aligned; even small decentrations on the order of a few microns will can cause deleterious degradation of the optical design performance.
Decentrations for the manufacture of aspheric objective lens assembly elements should allow several microns. Similarly, decentration of one aspheric objective lens assembly element with respect to another objective lens assembly element during assembly can also cause rapid degradation in image quality. Sensitively of this type is alluded to in the aforementioned article by Kiyoshi Osato et al. For the best manufacturable designs, these sensitivities must be minimized and accounted for. For two-element objective lens assembly designs that require three or four aspheric surfaces to achieve some target levels of performance, options for desensitizing the design to the aforementioned objective lens assembly element and manufacturing sensitivities are limited.
Objective lens assemblies are also known in the art, especially in the field of microscopy, that overcome some of the aforementioned deficiencies. Such an objective lens assembly is found in U.S. Pat. No. 4,418,988. These objective lens assemblies may have high numerical apertures and aberration-correction over finite field angles and typically possess larger working distances than those objective lens assemblies used in the optical recording field. However, these objective lens assemblies have limited applicability to the field of optical recording because of their large size and mass; large objective lens assemblies are not conducive for quick movement to accommodate rapid refocusing by a drive mechanism. Additionally, these objective lens assemblies often contain many more objective lens assembly elements and surfaces that make manufacturing difficult and expensive. Specific lens design examples illustrating the close relationship of microscopy lenses and optical recording lenses are found in the article xe2x80x9cMicroscope Objectives and Their Evolution to Optical Disk Objectivesxe2x80x9d, presented by Barry G. Broome of Optical Research Associates, and found in the proceedings of the SPIE Annual Meeting, January, 1992.
Presently, it is believed that there are no commercially available lens assemblies that meet the requirement of having a 0.65 or greater numerical aperture, large field, long working distance, and light mass.
It is therefore an object of the present invention to provide an optical objective lens assembly for focusing light that has a diffraction-limited high numerical aperture of at least 0.65, a large angular field, and a large working distance of at least 0.25 mm.
It is another object of the present invention to provide an optical objective lens assembly having a mass of less than 1 gram, and which is relatively easy to manufacture.
These objects are achieved by a high numerical aperture objective lens assembly for focusing light, comprising:
a) a first objective lens assembly element of negative optical power intercepting said light;
b) a second objective lens assembly element of positive optical power disposed adjacent to said first objective lens assembly element;
c) a third objective lens assembly element of positive optical power disposed adjacent to said second objective lens assembly element; and
d) wherein said first, second, and third objective lens assembly elements, in combination, include six surfaces wherein three or more of such surfaces are aspheric surfaces arranged so that the objective lens assembly has a numerical aperture equal to or greater than 0.65.
The present invention is particularly suitable for use in optical recording and readout systems since it is relatively easy to manufacture, has a low mass and, quite importantly, has a numerical aperture equal to or greater than 0.65. Moreover, the present invention provides for a large field and long working distance.